Why Did My Java Service Freeze? A Deep Dive into CPU Spikes, GC, and Debugging Tricks
The article recounts a production incident where a Java service’s CPU surged to 1200%, detailing how top, jstack, and jmap were used to trace the issue to a missing parameter that caused a full‑table scan, prolonged GC pauses, and ultimately a system freeze.
On a quiet morning the author was alerted that the offline system could not log in, and the production environment showed CPU usage up to 1200%.
Initial investigation used commands to identify the Java threads consuming CPU:
top -H -p <pid> # view Java threads with highest CPUjstack <pid> > xxx.txt # dump thread stackjmap -heap <pid> # inspect heap memoryScreenshot of the top output:
Further analysis of the access log revealed that a particular API endpoint was blocked for over a thousand seconds: localhost_access_log When reproducing the issue, the system froze again when opening a detail page, and CPU spiked again.
Investigation showed that the problem was not a high‑concurrency interface but a full‑table scan caused by a missing parameter in the business code, which forced the database to scan millions of rows, leading to prolonged GC pauses and CPU saturation.
Key take‑aways:
When CPU hits 100 %, capture a thread dump with jstack to locate the offending thread.
Use the access log to find unusually long‑running API calls.
Missing request parameters can trigger full‑table scans and severe performance degradation.
Even with ample memory, GC pauses can block the application for minutes.
Additional screenshots illustrate the monitoring output and GC pause:
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